Note: Descriptions are shown in the official language in which they were submitted.
3~
The present invention relates to a method of producing
metal by electrolysis in a molten salt bath.
The present invention may be thought of as an improve-
ment on the practice of using a glass barrier in the manner set
forth in United States Patent Nos. 3,773,643 and 3,779,699 of
A.S. Russell and L.L. I~napp for "Furnace Structure", issued
respectively on ~ovember 20, and December 18, lg73. These
pa-tents point out that it can be e~tremely dificult to contain
the molten salt baths used for producing metal by electrolysis of
1 aluminum chloride in bipolar cells. It has been discovered tha~,
while the glass barrier will usually effectively contain molten
salt baths, it is nevertheIess possible occasionally for the
baths to leak through the barrier, for instance around the edges
of the individual glass sheets making up the glass barrier, or
through cracks arising in the glass.
As indicated in these patents of Russell and Knapp,
leakage of the molten salt bath into contact with the steel
container of a cell can cause evolution of a substance such as
chlorine at anodic locations. It is possible for this chlorine
to quickIy eat a hole in the steel. At cathodic locations,
fingers o~ the metal being produced can grow from the steel
inwards along the crack to lead to costly short circuiting of at
least a portion of the bipolar cell. In this connection, it has
been found in practice to be almost impossible to absolutely
isolate the steel outer shell of a cell from being able to engage
in the carrying of electrical current, once either the anode or
cathode of the cell finds a way to the shell, or instance
through molten electrolyte in a crack.
In view of the problems outlined in the background of
the invention, it is an object of the-present invention to
provide a method of producing metal by eIectrolysis in a molten
salt bath, in a bipolar cell, which method is improved by its
., -,~Y~l
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ability to resist leakage of electrical current through molten
salt away from the cell.
This as well as other ob~ects which will become ap-
parent in the discussion that follows are achieved, according to
the present invention, by providing a method including producing
metal by electrolysis in a molten salt bath, in a bipolar cell,
wherein the improvement includes electrically isolating the bath
with a continuous, electrically insulating material in the
portion of the cell containing -the bath, which material is a
plastic or rubber.
Plastic or rubber containment o a bipolar cell in
a~ueous electrolytic systems has been practiced. Examples are
the following United States Patents:
Patent No. Inventor Issue Date
Re. 26,644 Forbes Aug. 19, 1969
3,287,251 Horne et al. Mov. 22, 1966
Apparently because of high temperatures involved in the electro-
lytically active portions of the ceIls, no one has ever thought
of applying this technology of aqueous cells for molten salt
cells. Even in the case of the cell tnot a bipolar cell) in U.S.
Patent No. 3,372,105 issued March 5, 1968, to A.F. Johnson for
"Aluminum ~eduction Cell and Insulation Material Therefor", where
a plastic sheet is used, the actual electrical insulating during
cell operation is done by a thin, dense layer of size-graded,
electrically non-conductive, refractory mineral particles. The
normal operational voltages that need be insulated against in a
monopolar cell such as in 3,372,105 are relatively small as
compared with those in the present invention concerned with
bipolar cells where the normal operational voltage will in
general be at least 10 volts.
An idea included in a narrower aspect of the present
invention is to use cooling to bring the temperature in the walls
of the cells -to a level such that the particular plastic or
rubber used should not be harmed by being subjected to a tem-
perature higher than it is able to withstand. For example, the
above-mentioned 3,773,643 and 3,779,699 mention water cooling of
the cell. Further examples are ~he following United States
patents:
Patent No. Inventor Issue Date
881,934 von Rugelgen et al. March 17, 1908
2,783,195 Ray~es et al~ Feb. 26, 1957
Care must be exercised, however, to make sure that the cooling
actuallv gets to the plastic or rubber, because otherwise the
material burns or carbonizes. This cooling o~ -the plastic or
rubber may be achieved, for example, by directly bonding the
material, as a coating, to a cooled metal cell container.
The present idea of using plastic or rubber has been
tried in ceIls of processes such as illustrated in United States
Patent No. 3,822,195 issued July 2, 1974, in the name of Dell et
al. for "Metal Production" and it has been found that it accom-
plishes the objects nicely. In general, resistance measurements
between (1) either of the bus bars connected to the anode and
cathode and (2~ a steel cell container coated with rubber or
plastic according to the invention will read at least 2 ohms
during process operation. Preerably, the resistance is at least
25 ohms and more preerabIy at least 40 ohms.
The present invention is particularly important in the
case of a bipolar cell contained in a metal shell, for instance a
steel shell, producing anode product, for instance chlorine,
which eats through the metal on contacting it. Thus, if bath
gets into contact with the metal sheIl, the fact that the cell is
being operated in bipolar fashion means that a relatively large
voltage is available for participation of the shell itself in an
electrolysis. ~n practice, it has been found that, despite all
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measures to pre~ent it, the shell of a cell such as that in
3,822,195 generally assumes the voltage of the anode. In this
case, almost the entire voltage across the cell may be brought
into generating nascent chlorine on the steel shell. This
nascent chlorine reacts essentially quantitativel~ with the
steel, causin~ rapid perfor~tion. Should perforation occur,
coolant, such as water, ~rom the cooling of the cell can enter
into the molten bath where it reacts sometimes violently.
~ven where it has been possible to completely isolate
the shell, molten bath need only get into contact with the shell
at two different locations and then up to almost the entire
voltage across the cell can come into action again, producing
chlorine at one location and metal at the other location.
It would be possible to make the walls of a cell thick
enough such that the plastic or rubber could be placed far enough
away from the electrolytically active region that the temperature
existing at the location of the plastic or rubber could be below
; its maximum service temperature, without it being necessary to
-Eorce the fall of temperature in the cell walls by, for instance,
2a water cooling. ~owever, there is a practical limit to this, i.e.
thé cost of building very thick walls on a cell must be balanced
against tha cost of using a techni~ue such as water cooling.
E'igure 1 is a sectional elevation of a portion of a
cell for producing metal in accordance with the invention. Since
this invention represents an improvement regarding just the walls
of a cell such as that disclosed in U.S. Patent No. 3,893,899,
this Figure 1 focusses just on thb improvement, steel side 12 in
this Figure 1 corresponding to steel shell 1 in 3,893,899 and
brick 24 here corresponding to ~rick 3 there.
Figure 2 is a schematic representation showing moni-
toring of the present invention.
A portion o-E a cell for electrolytically producing
aluminum by the electrolysis of aluminum chloride dissolved in a
molten salt bath utilizing the present invention is illustrated
in the ~rawing. The cell structure inclu~es an outer steel
cooling jacket 10, which surrounds the steeI sides 12 of the
cell. A cooling fluid ~coolant), for example water, flows
through jacket 10 for withdrawing heat from the cell. A struc-
tural containment 18, for example of steel, encloses and supports
the ceIl and the cooling jacket.
The bath-'containing cell interior surfaces, i.e. those
formed by sides 12 and a corresponding steel bottom, are lined,
in accordance with the presen~ invention, with a continuous,
corrosion-resistant, el'ectrically insulating lining 22 of plastic
or rubber material. Good results have been obtained with a
lining 22 composed of alternating layers of thermosetting epoxy-
based paint and glass fiber cloth. Other plastic or rubber
materials are possible. Thus, included as candidate plastic or
rubber materials for application in the present invention are
both the natural plastics such as asphalt and the synthetic
plastics such as polytetrafluoroethylene, silicone resins, and,
in general, epoxy resins. The rubber materials include both
natural and synthetic rubbers also. Various fillers may be used,
including fibrous reinforcements such as glass fibers. Also
present may be, for example, antioxidants, heat stabilizers, and
plasticizers. The particular plastic or rubber composition used
will be seIected taking into consideration, for example, the
temperature to exist at its location, the manner of putting it in
place, and the intended life of the cell.
Inwards of the lining 22 is interposed a glass barrier
13. For further informatlon concerning this glass barrier, see
the above-mentioned 3,773,643 and 3,'779,699. The cell is also
linea with'refractory side wall brick 24, made of thermally
insulating, electrically nonconductive, nitride material which is
resistant to a molten aluminum chloride-containing halide bath
and the decomposition products thereo~ (see U.S. Patent No.
3,785,941 issued January 15, 1974, in the name of S.C. Jacobs for
"Refractory for Production of Aluminum by Electrolysis of Alu-
minum Chloride").
Referring now to E~igure 2, there will be illustrated an
example of how the monitoring of resistance can be accomplished.
Bipolar cell 30 is provided with a sidewall and bottom construc-
tion as illustrated in Figure 1 and has associated with it an
anode bus 32 and a cathode bus 34. The desired resistance
measurement of coating 22 (Figure 1) may be achieved simply by
connecting resistance meter 36 between any arbitrary point 38 on
the steel shell 12 (Figure 1) and, in the example illustrated
here, anode bus 32. The resistances other than the desired
resistance of coating 22 are in general sufficiently small that
it is the resistance to flow of electrical current through the
coating 22 that is being measured. However, with this connection
of the resistance meter, it does not matter whether the glass
barrier 13 (Figure 1) or the brick 24 (Figure 1) would be con-
tributing major resistances to the flow of electrical currentthrough the coating 22, the important thing being that a resist-
ance is present. Furthermore, whether connection is made to the
anode bus or the cathode bus makes only a negligible difference
due to the conductivity of the molten salt. It can furthermore
be advantageous to connect the other side of the resistance meter
to jacket 10 (Figure 1), since jacket 10 will in the usual case
be in direct contact at the ends of the jacket with shell 12. A
suitable resistance meter is that disclosed in E.J. Seger et al.
U.S. Patent 4,188,267 issued February 12, 1980 for "Method of
Measuring the Integrity of an Electrolytic Cell Lining". The meter
can be in operation continuously during the time that electroly-
sis is being carried out in cell 30, and an alarm 40 can be set
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to produce a warning signal and e.g. turn off electrolytic
current supply 42 when the resi~tance measurement falls below
e.g. 2 ohms.
Further illustrative of the present invention is the
following example:
'' Exam~le~
The steel' sheIl formea by sides 12 and bottom (not
shown~ was sand blasted on its inner surfaces to remove mill
scale, rust, oxide, etc. and then blown free of all foreign
particles with dry air. The inner surfaces were`then provided
with'four epoxy paint coats, and, interposed between each two
adjoining epoxy paint coats, an epoxy paint coat with glass fiber
cloth pressed in. This makes a total of three epoxy paint coats
containing glass cloth plus the four plain epoxy paint coats
e~uals seven coats directly bonded to the steel shell. The total
; thicknes's caused by these seven coats was one-eighth of an inch.
The'particular epoxy paint used was National Electric Coil
Company ZA440 Thermopoxy Paint. Every coat of epoxy paint is
applied to a wet thicknes's of 8 mils. The ZA440 Thermopoxy Paint
is a two-part paint system re~uiring mixing of a base and a
catalyst activator together. The'mixing was done with an elec-
tric powe'rea paint mixing paddle until both base and activator
were thoroughly mixed, the mixing ratio for the two-part system
being 1 part catalyst to 7 parts epoxy base, the parts being on a
weight basis. Pot life'of the mixed two-part system is 20
minutes', and only that amount of paint which could be mixed and
applied within 20 minutes was applied at one time. The paint was
applied with a paint roller and pan. The paint coat becomes
tack-free in 4 to 8 hours. The complete cure of each paint coat
requires 96 hours at room t'emperature. Curing time was accel-
erated by circulating hot water through jacket 10. Additional
coats of paint we're not applied until the preceding layer had
completely cured. Following completion of the first plain coa~
of epoxy paint, a second coat of epoxy paint was applied to a wet
thickness of 8 mils and into this second coat was pressed glass
cloth. The glass cloth is pressed in while the paint is still
wet. A roller is usea to work out all wrinkles and air bubbles
in the cloth. Curing is then effected. ~ollowing this comes a
second coat of plain epoxy paint. This is cured and followed by
the'second coat of epoxy paint with glass cloth pressed into it,
and so on. Abutting sections of glass cloth in any given coat
were overlapped. $he'ceIl had holes' in its sides for reception
of ceramic tubes' containing the'anode'and cathode leads. The
same'layered plastic coating was provided on walls of khese
holes. The gap between the holes and the ceramic tubes was
finally packed tightly with ceramic fiber rope. See U.S. Patent
3,745,106 issued ~uly 10,,1973, in the name of S. C. Jacobs for
"Fluid Sheathed Electrode Lead for Use in a Corrosive Environ-
ment". The'remainder of the ceIl was constructed as a 12 com-
partment bipolar cell (i.e. an anode, a cathode and 11 bipolar
eIectrodes) and then filled with an average molten salt bath of
0 the following composition in weight percent:
NaCl 51.0
LiCl 40.0
AlC13 6.5
MgC12 2.5
The walls of ceIls working with this composition plus naturally
occurring impurities have been ~ound, upon autopsy, always to
conkain salt compositions which remain molten at temperatures
below 120C and usually are at least partially molten at room
temperature. Electrolysis to produce molten aluminum and chlo-
rine was carried out with 31 volts across the ceIl and an averagetemperature of 715C. During cell operation, resistance between
bus and side 12 lay in the range between 40 and 100 ohms. I~ has
been found that this variation is caused by things such as dust
on the outside of the steel shell extending over the shell to the
bus, so that holes in the plastic coating are not indicated by
such variation. Only when the resistance falls perhaps below say
2 ohms need one really begin to get worried about holes in the
coating.
* * * *
Various modifications may be'made in the invention
without departing from the spirit thereof, or the scope of the
claims, and, therefore, the exact form shown is to be -taken as
illustrative'only and not in a limiting sense,' and it is desired
that only such limitations shall be placed thereon as are imposed
by the'prior art or are-'specifically set forth'in the appended
claims.
